The present invention is directed to onboard fuel vapor recovery systems employed on motor vehicles to prevent or minimize the discharge of fuel vapor from the vehicle fuel tank into the atmosphere. The head space in the fuel tank of a motor vehicle must be vented to atmosphere to equalize the pressure in the head spaced as fuel is withdrawn from the tank by operation of the vehicle engine.
For many years, this venting was accomplished by a simple vent in the closure cap of the fuel tank fill pipe; more recently, this venting is performed through a charcoal filled vapor canister vented to atmosphere and also connected via a socalled purge system to the intake manifold of the vehicle engine so that fuel vapor can be withdrawn from the canister at a controlled rate for combustion in the engine. The rate at which vapor is withdrawn from the canister for combustion by the purge system is carefully controlled to a limited rate to avoid overly enriching the normal fuel mixture, and the rate at which pressure equalizing flow from the vent into the tank occurs to compensate for fuel consumption is minimal. Thus, these systems employ relatively restricted flow passages to minimize the possible discharge of fuel vapor from the canister vent.
Such systems are totally inadequate to cope with the massive surge of fuel vapor displaced from the fuel tank by incoming fuel during a refueling operation and the vapor displaced during refueling is normally discharged into the atmosphere through the fill pipe inlet.
In a commonly owned co-pending application Ser. No. 07/101,069, filed Sept. 25, 1987, there are disclosed vapor recovery systems designed to recover and store fuel vapor displaced from the tank during the refueling operation. In essence, these last systems employ two canisters connected in parallel with each other between the fuel tank and purge systemnamely a relatively small "running vapor" canister vented to atmosphere and connected to the fuel tank at all times via a relatively restricted conduit (the system described above), and a relatively large "refueling vapor" canister connected by a relatively unrestricted conduit to the fuel tank only while the vehicle is being refueled. The systems described in the aforementioned co-pending application utilize a refueling vapor valve opened either electrically or mechanically in response to the insertion of a service station fuel pump nozzle into the fill pipe to place the tank in communication with the large "refueling vapor" canister.
When the nozzle is removed at the conclusion of the refueling operation, the valve automatically closes. The refueling vapor system also includes a float valve responsive to the level of fuel in the tank which will closes to disconnect the refueling vapor recovery system from the tank when the fuel level in the tank rises to a predetermined level.
The present invention is directed to a refueling vapor recovery system which may, if desired, employ only a single canister connected to the head space of the fuel tank in a manner such that substantially unrestricted flow of vapor from the tank to the canister can occur during a refueling operation while the flow of vapor from the tank to the canister is limited or restricted at all other times. The system utilizes a single pressure responsive valve of relatively compact construction which may be mounted directly upon the fuel tank of the vehicle.